In numerous applications of a closable member, such as a door hinged in a frame, for example, in some electric locking mechanisms, it is a requirement to provide passage for electric power between the door and the frame. Fundamentally, this requires a bundle of flexible and extendable wire conductors to bridge across the gap between the door and the frame. Because the gap changes in both length and direction as the door swings on its hinges, a wire bundle is subject to damage by becoming pinched, abraded, or sheared. Further, a wire bundle is easily severed maliciously by persons wishing to defeat the mechanism. Thus, substantial prior art is devoted to systems for providing movable protective shielding between a door and frame to prevent damage to a wire bundle passing therebetween.
All such movable systems are confronted with a common set of geometric requirements.
First, the system must be accommodated within the door and/or frame when the door is closed.
Second, the system must be capable of extension and retraction between the full closed and fully open door positions.
Third, the system must be capable of pivoting between the system ends through an angle equal to the open angle of door in the frame, i.e., if the door is opened 180°, the system must also be capable of rotating 180° within itself from its starting configuration.
Fourth, the ends of the system at door and frame must each be capable of rotational motion in at least two dimensions, i.e., in planes parallel and orthogonal to the planes of the door edge and the frame edge, respectively.
U.S. Pat. No. 4,445,299 discloses an arrangement comprising first and second members mountable to a door and frame. Arranged around an electrical cable is a separate, turnable, and longitudinally flexible protecting element such as a coil spring. This element is arranged basically in the longitudinal direction of the door frame. One end of the spring is connected to the door frame member and the other end to the door. There is in the door frame and/or in door edge part at least one recess so arranged that, when the door is closed, the protecting element and cable are received in this recess.
Although simple in construction, the device has at least two important shortcomings. First, the spring acts as a torsion spring to exert an opening or closing force on the door as the spring ends are counter-rotated, which may not be desirable. Second, when the spring is flexed and extended as the door opens, the spring coils are spread apart, thus allowing easy insertion of a sharp tool as might be desired for malicious severing or shorting of the electrical wiring.
U.S. Pat. No. 5,690,501 discloses an apparatus for providing an electrical connection from a power source, through a stationary door frame, into a door to connect to door-mounted electrically-operated hardware, including an electrical junction box installed in a mortise cavity in the door frame adjacent to a similar electrical junction box installed in a mortise cavity in an edge of the door. The junction boxes are adjacent each other when the door is in a closed position, and a housing is installed in each junction box, each housing having a longitudinal slot in a face plate thereof of a length substantially equal to the length of the junction box. A cover plate is longitudinally slidably engaged in grooved fingers on a rear surface of the face plate, the cover plate having a substantially circular hole. A hollow tube, having an elbow swivelably mounted on each end, has one elbow swivelably engaged in the substantially circular hole of the cover plate of the housing mounted in the door frame and the other elbow swivelably engaged in the substantially circular cover plate of the housing mounted in the door. A conductor cable connected to a power source, in the junction box mounted in the door frame, extends through the elbows and the hollow tube into the junction box mounted in the door, and connects to leads to door hardware mounted in or on the door. In operation, as the door is opened, because the length of the elbows and hollow tube is constant the cover plates are caused to slide toward each other as the tube rotates within the elbows.
This device has at least two shortcomings. First, it requires a relatively large number of components and thus is relatively expensive to manufacture. Second, as the cover plates slide toward one another, the force vector for return of the plates becomes progressively less favorable; thus the plates potentially can become cocked and jammed in their guides, especially as the mechanism becomes worn with extended use.
What is needed in the art is a simple, reliable apparatus for transferring electric power between a door and a frame wherein the number of components is relatively low, the manufacturing cost is relatively low, and the wiring bundle is fully protected at all times.
It is a principal object of the present invention to reduce the cost and complexity of an apparatus for transferring electric power between a door and a frame.